1. Field of the Invention
The invention relates to a method for the manufacture of intraluminal stents of bioresorbable polymeric material.
2. Background Art
As regards the technological background of the invention, it has to be explained that for the manufacure for instance of implantable intravascular stents--i.e. of angioplastic vessel wall supports to be used in cardiac surgery--conventional methods of plastics technology are known and used, which are based on the processing of thermoplastic polymeric materials. Stents, which are small tubes of some few millimeters of diameter and few centimeters of length, are produced by extrusion or injection-molding. In this connection, reference must be made to the essay, "Evaluation of poly (L-lactic acid) as a material for intravascular polymeric stents", by Agrawal et al., in Biomaterials 1992, vol. 13, no. 3, pages 176 to 187, mentioning the use of poly (L-lactic acid) monofilaments for the manufacture of intravascular polymeric stents. These monofilaments are extruded and drawn to different draw ratios. Monofilaments thus treated are then used for the construction of stents.
The disadvantage of the known methods resides in that they may lead to the polymeric material being thermally damaged during the extrusion process or injection-molding. The high crystallinity of the materials used can make thermoplastic transformation processes very difficult.
The essay, "Fabrication of Resorbable Microporous Intravascular Stents for Gene Therapy Applications", by Rajasubramanian et al, ASAIO Journal 1994, pages M584 to M589, teaches to manufacture resorbable, microporous stents from a mixture of poly-L-lactic acid (PLLA) and poly-E-caprolacton (PCL), spiral as well as tubular stent constructions being produced by dissolution of this polymeric mixture in an organic solvent (1,4-dioxane) and by subsequent flotation of the polymer. During this flotation, the polymer solution is sprayed on a water surface flowing steadily, the solvent thus dispersing in the water and evaporating from the surface. This helps form a polymer precipitation floating as a film on the water surface and being taken up, at a suitable place, by a mandrel, on which will form a multi-layer coating of the partially cured polymer. After completion of the coating, the mandrel is placed into a vacuum furnace for the curing process to be completed over a period of 24 hours at 45.degree. C. Then the mandrel comprising the polymer coating is soaked in a 50 per cent ethanol solution for the polymer coating to swell, after which it can be removed from the mandrel.
The known method poses problems in as much as the flotation of the polymer film and the "take-up" of the polymer film by the mandrel is very difficult and susceptible to process fluctuations. Moreover, there is the need of complicated subsequent treatment of the stent blanks thus produced. Another disadvantage of this method resides in that no isotropic structure within the stent can be attained by this manufacturing technology. This is of disadvantage for any steady resorption in vivo and negatively affects the mechanical properties of the stent.